Everything You Need to Know About Colour Management

Colour Management and nature (…but were afraid to ask)

You’ve probably often heard about colour management, but wondered why you need to manage your colours. You’ve probably heard of sRGB and AdobeRGB, but wondered what difference they made. You’ve possibly heard about Colour Space, but ignored it because it’s sounds like something techy you don’t need to worry about. Well, the good news is that these are all easy things to understand.

Colour is funny stuff

For years, people have told you that there are 256 or 16,384 shades of Red, Green, and Blue on a computer display, making up all the colours you’ll ever need. This is commonly known as 8-bit or 16-bit RGB. This is really all you need to know about colour. These people have been lying to you.

The computer colour system you have been using all these years is called sRGB (standard RGB), and it was only named in 1996. sRGB is one way of translating three values of “Red”, “Green”, and “Blue” into different colours. A pixel in a digital image contains three numbers, and this “code” is translated to colour you can see. You know all this; why am I telling you again? Notice I said “one way”. There are other ways of translating RGB pixels into colour too…

If you could print every colour available in sRGB accurately on a sheet of paper, and compare them with real objects, like flowers, you might notice that these objects contain colours missing from your sheet. There’s a good historical reason for this: when sRGB was invented, scanners, printers and monitors were limited in the colours they could reproduce (and most still are), so there seemed little point in having a code for a colour they couldn’t make.

Moving goalposts

Adobe produced its own version of RGB in 1998. The idea was to expand on the palette of colours in sRGB, and include the colours available in cutting-edge printers too. Again, your pixels have three values of “Red”, “Green”, and “Blue”, but these base colours are different, and so the end result is different. AdobeRGB translates the numbers into a wider range of colours than sRGB, with noticeably deeper hues.

The latest digital cameras can actually record colours well outside those available in sRGB and AdobeRGB colour spaces. Kodak invented ProPhotoRGB to include these colours, but it isn’t yet widely supported. As technology progresses, devices (like monitors, printers, and projectors) will support a wider range of colours, and ProPhotoRGB will come into its own — much like AdobeRGB did, 10 years after its introduction.

Make your mind up!

Oh dear, this is starting to sound tricky: “different results?!” It’s not a big deal. Whenever you provide a digital image, it just needs to be tagged with the name of the colour system used — this is that Colour Space you keep hearing about. Your photo software does this automatically. See, it’s easy, isn’t it? Every colour image has an associated Colour Space (sometimes called ICC Profile), and the image only contains colours dictated by that Colour Space.

Think of a Painting-by-Numbers book. The numbers in the pictures correspond to the number in the paint box. Mix up two different books with two different paint boxes, and you could have some funny-looking paintings. You must keep the paints with the right books. Similarly, a digital image must be marked with its Colour Space, or you risk getting it wrong.

Shoe-horning colour

Many newer monitors, printers, and projectors support colours outside the sRGB colour space. If you want to show off that red rose, lush meadow, or heavenly sea in your photos, you need to use the best tool for the job — which means choosing your Colour Space appropriately.

JPEG shooters should be using the biggest possible colour space available (usually AdobeRGB) if they think they will ever need to display more colours than those available to sRGB monitors and projectors. Maybe, in 10 years’ time, all display devices will be AdobeRGB-compatible. It’s fairly likely. Those sRGB photos will look a bit dull by comparison.

For now though, AdobeRGB images usually need to be converted to sRGB for reliable display. This means losing colour. Generally, there are two ways to do this: a Perceptual conversion squashes all the colours together to fit, giving the best variety of colour, but compromising accuracy; and a Relative Colorimetric conversion picks the nearest colours to the “lost colours”, just inside the new colour space, giving better accuracy for all but the out-of-range colours.

Thinking outside the box

So, although starting with the best Colour Space will guarantee that you can future-proof your images, shooting Raw neatly side-steps the whole issue — Raw files effectively have no Colour Space — the Colour Space is applied only when the Raw data is converted into a digital image.

A digital image with a big Colour Space stands a better chance of producing a good print on an expensive printer, and will look better on a so-called wide-gamut monitor or projector, when (not if) you (inevitably) upgrade.

Keeping it all under control

That’s all there is to Colour Management. Sorry, I lied. Much current software is like you might have been at the start of this article — unaware of the different types of Colour Space. Windows’ Internet Explorer and Picture & Fax Viewer are two popular applications which don’t properly understand colour space, and assume all images are sRGB images and/or all monitors are sRGB monitors. But, hopefully, you use colour-managed software (like Photoshop) and you calibrate your monitor, printer, projector and camera
What do you mean, “no”?

Keith Nuttall, 2009 (revised 2011)

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